The rate constants, the free energies (ill;#l. the energies (LliJ#) and entropies (t.s#) of activation, and the steric parameter of substituent (Es), for both the acid and the alkaline hydrolyses of four highly branched ethylene carbonates (la -d), and 11 trimethylene carbonates (lia -k) were analysed according to Taft's procedure for a quantitative separation of polar and steric effects of alkyl substituents in the total effect of structure upon reactivity in hydrolysis. Application of Eq. (II) with a* to the most hindered substrates ("a-axial methyl effect") (lId -0 and to the moderately hindered substrates (lla -c) gave well-separated parallel straight lines for the two groups. Deviations from Eq. (II) were manifested by the rate-enhanced group of compounds, gerninally 2,2-disubstituted trimethylene carbonates (llg -k) ("2,2-Kem-dialkyl effect"), shown to exert a polar "field effect"; the assessment of its substituent constant aF· (= +0.17) is provided by Eq. (III). Analysis of 0 18 exchange experiments and MFA #vsMFB# correlation (Eq. (IV)) show that the acid-and base-catalysed reactions occur in parallel via similar transition states, the attainment of each designating the rate-determining step in both systems. A value of 0.5 kcal/mole was found for the net steric interaction (EgB -EgA) exerted by an a-axial methyl substituent on the attacking hydroxide ion in attaining the transition state. It was concluded that in alkaline hydrolysis, the transition states of compounds exhibiting an "a-axial methyl effect" should closely resemble the tetrahedral intermediate (IX), while the transition states of the 2,2-gem-dialkyl series should more closely resemble the ground-state.